1. Field of the Invention
This invention relates generally to a color video camera providing a digital video signal and having auto-focus, auto-exposure and auto-white balance functions, and further is directed to an exposure control system particularly suited for incorporation in such camera and which compensates for abnormal lighting conditions.
2. Description of the Prior Art
It has been known to provide a color video camera with analog auto-focus, auto-exposure and auto-white balance control systems. However, such analog systems are susceptible to change with variations in temperature and, therefore, do not provide the desired high degree of accuracy and reliability. Although digital control systems do provide the requisite accuracy and reliability, the incorporation of digital auto-focus, auto-exposure and auto-white balance controls in a color video camera has involved relatively bulky circuit arrangements which are unsuited for incorporation in the miniaturized or compact hand-held video cameras now provided for the consumer market.
The auto-exposure control system embodied in a conventional video camera controls the opening and closing of an iris and the gain of an auto gain control (AGC) circuit with the object of maintaining the luminance signal level in an exposure detection area of the CCD imaging device at a predetermined value. With such conventional auto-exposure control system, back-lighting of a scene in the field of view of the camera causes a large increase in the brightness level of the background and, in response thereto, the system closes the iris and decreases the gain of the AGC circuit so that an object in the foreground of the screen appears dark and inconspicuous in a picture or image displayed from the resulting video signal. On the other hand, in the event of excessive forward-lighting of the scene, the brightness level of the background is much smaller than the brightness level of objects in the foreground and, in response thereto, the conventional auto-exposure control system tends to open the iris and increase the gain of the AGC circuit with the result that objects in the foreground of the picture are saturated.
In connection with the foregoing, it has been suggested to define two exposure detection areas which respectively contain the background and the foreground objects, so as to perform center-emphasized photometry under back-lighting or excessive front-lighting conditions with the object of controlling the opening and closing of the iris and the gain of the AGC circuit so that the luminance signal level in the exposure area that contains the objects in the foreground will be maintained at a predetermined level during back-lighting and excessive front-lighting.
When auto-exposure control is performed with reference to two exposure detection areas which respectively contain the background and the foreground objects, precise detection of the lighting condition, that is, whether the scene is subjected to normal front-lighting, back-lighting or excessive front-lighting, is required. However, in the previously proposed arrangement employing two exposure detection areas, as aforesaid, only integrated values of the luminance signal levels in the respective areas are obtained and employed to determine the lighting condition. As a result, precise detection of whether the scene is being subjected to normal front-lighting or to abnormal lighting, such as, back-lighting or excessive front-lighting, is not realized.